Generally, polyaniline is prepared in the form of base type polyaniline by reacting an aniline monomer with an oxidizer in the presence of a hydrochloric acid aqueous solution or a sulfuric acid aqueous solution thus obtaining a polyaniline salt and then neutralizing the polyaniline salt with a base. The base type polyaniline which is a nonconductor is imparted with conductivity only when doped again with an acid. The conductive polyaniline thus prepared does not dissolve in most organic solvents and does not fuse even when heated, resulting in poor processability. Also, the preparation process of the conductive polyaniline generates a large amount of wastewater due to the aqueous solution treatment, undesirably causing serious environmental contamination.
In order to solve such problems, various attempts have been made. In this regard, there are first disclosed methods of doping polyaniline with camphor sulfonic acid thus dissolving the doped polyaniline in m-cresol because polyaniline doped with an acid having a simple structure such as sulfuric acid or hydrochloric acid is difficult to dissolve. In addition, methods of doping a polymer using various acids thus dissolving the polymer in m-cresol are disclosed, but the industrial use thereof is restricted because of the high toxicity of m-cresol. Furthermore, methods of attaching a substituent to an aromatic ring of aniline to increase solubility are disclosed. Although these methods increase solubility, problems of conductivity being decreased and the preparation cost being increased occur, and wastewater is still generated.
MacDiarmid et al. (A. G. MacDiarmid et. al., “Polyaniline: A New Concept in Conducting Polymers”, Synthetic Metals. 18 (1987), pp 285-290) describes the results of observing the changes in conductivity of polyaniline obtained by de-doping polyaniline synthesized in the presence of a hydrochloric acid aqueous solution thus preparing base type polyaniline and then doping the base type polyaniline with various acids. However, this method is disadvantageous because the kind of acid usable as a dopant is limited, and the there is a limited number of solvents able to dissolve the conductive polymer.
Polypyrrole is typically prepared by polymerizing a pyrrole monomer with an oxidizer in the presence of an acid aqueous solution. However, because this polypyrrole is not dissolved in water or an organic solvent, the industrial use thereof is difficult. Also, Lee Jin-Young et al. (J. Y. Lee D. Y. Kim, and C. Y. Kim, Synthetic Metals 74 (1995), p 103) describes polypyrrole which is soluble in m-cresol, tetrahydrofuran (THF) or dimethylformamide (DMF) using dodecylbenzenesulfonic acid (HDBSA) as a dopant. However, the preparation of this polymer is disadvantageous because the dopant only includes dodecylbenzenesulfonic acid, the solvent able to dissolve the polymer is merely exemplified by m-cresol, tetrahydrofuran (THF) or dimethylformamide (DMF), and the solubility of the polymer is also limited.
It is known that polymers such as polythiophene, poly(p-phenylene), poly(p-phenylenevinylene) and so on, which are obtained using the synthesis methods reported to date, seldom dissolve in water or organic solvents.
U.S. Pat. No. 5,567,356 discloses an emulsion polymerization method, but is problematic because an excess of dopant remains in the organic solvent layer and the conductivity is low. Also, Korean Patent No. 10-0633031 discloses a method of increasing solubility using a dopant mixture, but is disadvantageous because the organic solvent layer and the water layer should be separated from each other, a dopant mixture which is expensive should be used, and there is not a variety of kinds of dopants. Furthermore, when a dopant having a complicated structure is present in a large amount in the organic solvent layer, and is removed, a large amount of wastewater may be undesirably generated.
Conductive polymers have a large variety of uses in the industrial field. However, the conventional conductive polymer is difficult to apply to various end uses because it dissolves in a minimum number of a specific organic solvent only when doped with a specific dopant as mentioned above. The main reason why the conventional conductive polymer has the above problems is considered to be due to the large molecular weight of the doped conductive polymer.
Generally, the synthesis of conductive polymers is carried out in a solvent including a considerable amount of water. The conductive polymer thus synthesized is reported to have a weight-average molecular weight of at least 10,000.
Specifically, Mattoso et al. (“Controlled Synthesis of High Molecular Weight Polyaniline and Poly(o-methoxyaniline”, L. H. C. Mattoso, A. G. MacDiarmid and A. J Epstein, Synthetic Metals, 68 (1994) pp 1-11) describes polyaniline having a weight-average molecular weight of about 53,000, synthesized in the presence of a hydrochloric acid aqueous solution at 0° C., and polyaniline having a weight-average molecular weight controlled to the range of 36,900˜384,900 under conditions in which the temperature is changed to be lower than 0° and the reaction catalyst is changed.
Korean Patent No. 10-0373926 (patentee: Smart Teck Co. Ltd., Inventor: Lee Sung-Joo) discloses polyaniline having a weight-average molecular weight of at least 10,000, synthesized in the presence of an acidic aqueous solution.
E. J. Oh et al. (“High Molecular Weight Soluble Polypyrrole”, E. J. Oh, K. S. Jang and A. G. MacDiarmid, Synthetic Metals, 125 (2002), pp 267-272) describes polypyrrole having a weight-average molecular weight of 62,296, synthesized by reacting a pyrrole monomer and sodium di(2-ethylhexyl)sulfosuccinate with ammonium persulfate in the presence of an aqueous solution at 0° C.
Jang Kwan-Sic (doctoral thesis, Department of Chemistry of Myong Ji University Graduate School, 2001) describes polypyrrole having a weight-average molecular weight of 10,000 or more, synthesized in the presence of an acid aqueous solution.
Also, Korean Unexamined Patent Publication No. 2001-0112574 discloses polypyrrole having a weight-average molecular weight of about 13,441, synthesized through an oxidation reaction using dodecylbenzoic acid and polypyrrole at 0° C. in the presence of an aqueous solution as mentioned above by Lee Jin-Young et al.
Based on the aforementioned theoretical grounds, in order that the conductive polymer doped with various dopants be able to be efficiently dissolved in various organic solvents, there is a need to synthesize a conductive polymer which has a weight-average molecular weight remarkably lower than the currently typical level (weight-average molecular weight: 10,000 or more).